1. Technical Field
The present invention relates generally to power tools such as rotatable drills and power screwdrivers. More particularly, the present invention relates to a clutch arrangement that is actuated on the basis of the magnitude of the torque that is output from the first stage of a multi-speed transmission.
2. Discussion
Modernly, manufacturers of power tools have introduced rotary power tools that have variable speed motors in an attempt to provide the users of these tools with sufficient control over the output speed of the tool so as to permit them to perform diverse operations without resort to additional, specialized tools. Many of the tools that are commercially available include a three-stage, two-speed transmission that permits even greater control over speeds of these tools.
Typically, the known transmission arrangements have lacked a configuration that could produce a wide range of output speeds and torques that would permit the tool to perform diverse operations such as drilling holes with a large diameter hole saw, installing drywall screws or large diameter lag screws, and performing high-speed drilling operations. The single or dual speed transmissions that were generally employed in these tools typically did not have sufficient speed reducing capacity to permit these transmissions to be diversely employed as configuring these tools for high torque operations tended to impair their high speed performance. Furthermore, the rechargeable batteries that were employed in many of the early cordless rotary power tools were not well suited for use in low-speed, high torque operations due to the amount of energy that is consumed and the rate with which the energy is consumed by the power tool during such operations. Consequently, consumers were often forced to purchase two different rotary power tools, a medium-duty tool for “standard” applications such as drilling and fastening, and a heavy-duty tool having a low-speed, high torque output for more demanding tasks.
With the advent of the modern high capacity, high voltage battery, it is now possible to meet the energy demands of a power tool that is used in low-speed, high torque operations. There remains, however, a need in the art for a clutch arrangement which can be used throughout a relatively large speed reducing range.
Typical clutch arrangements permit the user of the tool to limit the torque that is output by the last stage of the tool's transmission. These clutch arrangements commonly employ a spring that biases two portions of the clutch into engagement. When the torque that is output by the transmission exceeds the predetermined clutch setting, the biasing force exerted by the spring is not sufficient to maintain the portion of the clutch in an engaged condition and as such, one of the portions of the clutch is able to rotate relative to the other portion of the clutch. The relative movement of these two portions effectively inhibits the transmission of torque to the output shaft of the tool.
The use of such clutch arrangements with a transmission having a relatively large speed reducing range is often times impractical for the simple reason that the biasing force that is exerted by the spring does not have sufficient range to permit the clutch portions to be properly engaged and disengaged over the entire speed reducing range. Accordingly, it is relatively common to supply several different sized springs with a clutch arrangement, necessitating that the user select and install an appropriately sized spring for a given task. While this approach has been effective, it is nonetheless time consuming and inconvenient.